Intermediate pressure wall stabilized gas lamp



p 18, 1962 w. c. LOUDEN ETAL 3,054,922

INTERMEDIATE PRESSURE WALL STABILIZED GAS LAMP Filed Aug. 26, 1959 fFF/C/ENCY'L 040mm; CfiAPAcrfp/srlc 200 400 600 Vow/v5 ZO/JD/NG-MTTS PEP cc.

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Their Ahto1-neu United States Patent ()fifice 3,654,922 Patented Sept. 18, 1962 3,tl54,922 INTERMEDIATE PRESSURE WALL STABELIZED GAS LAMP William C. Louden, South Euclid, and Kurt Schmidt,

Cleveland, Ohio, assignors to General Electric Company, a corporation of New York Filed Aug. 26, 1959, Ser. No. 836,199 6 (Zlaims. (Cl. 313-184) This invention relates to intermediate pressure inert gas discharge lamps for continuous operation as distinguished from gas lamps intended for pulsed operation and more commonly known .as flashtubes. It relates more particularly to a relatively long are gap inert gas discharge lamp utilizing an envelope of sintered transparent high density polycrystalline alumina and wherein the plasma is wall-stabilized.

The term plasma describes a characteristic condition of an ionized gas of which two specific properties are as follows:

(1) A relatively high degree of ionization, the extreme case being total ionization of all neutral gas particles.

(2) A tendency toward electrical neutrality resulting from practically equal concentrations of positive and negative charge carriers and leading to a condition where substantially no space charges exist.

One may distinguish between a plasma operating without thermal equilibrium and a plasma operating with thermal equilibrium. The former is found in low pressure discharges, for instance in the conventional low pressure fluorescent lamp wherein the mean electron energy is very much greater than the mean gas or ion energy. The latter is found in high or very high pressure lamps operating at high temperatures. Heretofore lamps operating with a plasma at thermal equilibrium or substantial thermal equilibrium have generally required either artificial cooling of the envelope as by means of water cooling or an air blast, or else have operated at very high pressures to achieve constriction of the arc in order to keep it away from the envelope walls.

In the specific case of a xenon arc lamp, the conventional lamp construction for obtaining ellicient generation of light has been to use very high pressures, for instance 10 to 5G atmospheres, low current, and high power concentration to achieve a small discharge cross section, that is a constricted are or plasma. Stabilization was then achieved by additional means such as magnetic fields or diaphragms to keep the are from shifting or whipping around due to convection currents and also to prevent it from contacting the envelope wall. By such means, the wall temperatures were limited to approximately 800 C., the maximum safe usable temperature for a quartz envelope.

More recently, relatively efficient generation of light from a xenon arc has been achieved by means of low pressures, for instance one atmosphere or less, high current, relatively low power concentration and large discharge cross sections. Such lamps operate with wall stabilization, the temperature again being limited to about 800 C. in the case of a quartz envelope.

The object of the invention is to provide new and improved gas are lamps operating at intermediate pressures, that is pressures in the range of l to atmospheres, and wherein the discharge or plasma fills the entire envelope cross section and wall stabilization is achieved.

Another and more specific object of the invention is to provide improved xenon arc lamps operating at intermediate pressures .and relatively high efficiencies by means of wall stabilization.

In accordance with the invention, we provide an inert gas lamp utilizing an envelope of sintered transparent high density polycrystalline alumina and which operates at intermediate pressures in the range of 1 to 10 atmospheres and which achieves wall stabilization without artificial cooling. The lamp operates at medium current and intermediate power concentration and with a relatively small discharge cross section. The discharge space of the lamp has an effective length, measured between electrode tips, several (at least 3) times its diameter and the diameter is such that the discharge or plasma fills the entire cross section. Wall stabilization under these circumstances is feasible by virtue of the high wall temperatures allowed by the high melting point of the sintered alumina, up to 1925 C. The gas filling may be essentially one of the inert gases xenon, krypton, argon, neon, helium or mixtures thereof. However xenon is preferred on account of its higher efi iciency due to the more favorable ratio of light output to heat losses, and on account of its desirable spectrum which results in a white light.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description of a preferred embodiment taken in conjunction with the accompanying drawing. The features of the invention believed to be novel will e more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a sectioned view of a xenon lamp utilizing a sintered polycrystalline alumina envelope and embodying the invention.

FIG. 2 illustrates certain operating characteristics of the lamp of PEG. 1.

Referring to the drawing, the lamp comprises an envelope l of ceramic tubing of sintered transparent polycrystalline alumina. The tubing material has a very high alumina content, in excess of 99.5% A1 0 and is highly transparent, that is, it has exceedingly good light transmittance, in excess of The term transparent is used herein in the sense of light-transmitting, and the tubing is translucent rather than clear like glass. Metal caps 2, 3 consisting of a nickel chromium iron alloy having a high temperature melting point and .a coefficient of expansion matching that of the alumina are brazed to the end of the alumina tube using thin titanium rings or washers to metalize the ends of the tube in order to bond the caps thereto.

Alumina back-up rings 4, 5, in effect short sections of tubing of the same diameter and wall thickness as the tubing 1, are brazed to the outer faces of the caps 2, 3, again through the use of titanium Washers to metalize the surface of the ceramic rings. The purpose of these rings is to balance any strains set up between the end caps 2, 3 and the alumina parts to which they are brazed throughout the temperature range to which the ends of the lamp are subjected in operation.

The brazing of the end caps is done in a vacuum or in a reducing or inert atmosphere. For instance, to braze the end cap 2 and ring 4 together and on to the end of the tubing 1, the parts are placed in a vacuum furnace and assembled in the following order: tubing 1, a first thin titanium washer, the nickel chromium iron alloy cap 2, a second titanium washer and then the back-up ring 4. The parts are pressed together and the furnace either evacuated or filled with a reducing or inert atmosphere and the temperature raised to approximately 955 C. Reference may be made to the copending application Serial No. 836,201 of William C. Louden and Elmer Homonnay, filed of even date herewith, entitled Metal-to-Ceramic Seal, and assigned to the same assignee as the present invention, for details of a suitable seal construction and the method of making it.

The end cap 2 at one end of the lamp is centrally perforated through an outwardly projecting embossment 6 through which is passed a stainless steel tube 7. The tube 7 is brazed to the cap 2 to make a hermetic seal and supports on its inner end a cathode 9 consisting of a double wound tungsten Wire coil with the interstices filled with activating material in the form of alkaline earth oxides including barium oxide. The tungsten coils forming the cathode are wound over a tungsten shank 10 which is pinched or welded in the end of the stainless steel tube 7. Electrode 11 at the other end of the tube is supported from a short length of stainless steel tubing 12 welded into the outwardly projecting embossment 13 of end cap 3, no perforation of the end cap or projection of the tube being required at this end of the lamp.

The tube 7 is used to evacuate the lamp and to introduce the xenon gas therein. The side aperture 14 in tubing 7 permits the passage of gas out of the envelope while exhausting, or into it while gas-filling. After evacuation of the envelope and filling with xenon, the tube '7 is pinched and welded as indicated at 15 to seal off the lamp.

Gas arc lamps inaccordance with the invention operate with wall stabilization at intermediate pressures in the range of 1 to 10 atmopsheres and with volume loadings in the range of 200 to 800 watts per cubic centimeter. Such high volume loading is made possible through the use of an elongated envelope of high density polycrystalline alumina which closely surrounds the discharge and. operates at temperatures in the range of 1000 to 1600 C. If shorter life and poorer lumen maintenance can be tolerated, the wall temperature may be increased towards but not beyond approximately 1925 C. which is the melt ing point of the alumina. The operating pressures of l to 10 atmospheres are achieved using cold (room temperature) filling pressures of inert gas from approximately 200 to 1250 millimeters of mercury.

The inert gases xenon, krypton, argon, neon and helium may be used, but, as previously mentioned, xenon is preferred on account of its higher light-generating eificiency and desirable color rendition. With xenon as the filling gas, efliciencies above 10 lumens per Watt are obtainable substantially throughout the volume loading range of 200 to 800 watts per cc. in accordance with the invention. In the upper portion of the stated range, efficiencies of lumens per watt are obtained and the volt-ampere characteristic is positive. Thus the added advantage is obtained that the lamp may be operated without a current limiting impedance, or with minimum impedance in series merely for the purpose of improving stability.

As an example of a lamp in accordance with the invention, in the lamp illustrated in the drawing the alumina tubing has an internal diameter of 6 millimeters and overall length of 10 centimeters. The distance between electrode tips is approximately 5.5 centimeters. At an arc current of 18 amperes, the lamp operated with an arc drop of 48.3 volts, the loading being approximately 810 watts which corresponds to a volume loading of approximately 600 watts per cubic centimeter. Under these conditions, the envelope temperature midway between the electrodes was approximately 1300 C., the operating pressure of the xenon gas was approximately 2.8 atmosphere, and the luminous etliciency was approximately 20 lumens per watt. The luminous elficiency versus volume loading characteristic of the lamp is illustrated in FIG. 2, the solid part of the curve showing measured results, and the dotted part showing the projection indicated by other tests. The relatively flat efficiency versus loading curve about 600' watts per cc. will be observed. The lamp, at this loading, operated with perfect wall stabilization, the discharge filling substantially the entire cross section of the envelope, and showed a positive volt-ampere characteristic.

Although the lamps in accordance with the invention operate at intermediate pressures of l to 10 atmospheres only, they operate with a plasma in substantially complete thermal equilibrium and which extends right up to the envelope walls. The substantially complete thermal equilibrium of the plasma is evidenced by the strong con- 4 tinuum in the discharge spectrum, the light emitted by the lamp being very similar to sunlight in quality.

The foregoing example of the invention is intended as illustrative and not in order to limit the invention thereto except insofar as specific limitations may appear in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An intermediate pressure gas are lamp comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of electrodes sealed into opposite ends, an inert gas within said envelope developing an operating pressure in the range of 1 to 10 atmospheres at volume loadings in the range of approximately 200 to 800 watts per cubic centimeter, the discharge space of said envelope having an effective length several times its diameter and providing wall stabilization.

2. An intermediate pressure gas are lamp comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of electrodes sealed into opposite ends, an inert gas at a cold filling pressure of approximately 200 to 1250 millimeters within said envelope developing an operating pressure in the range of 1 to 10 atmospheres at volume loadings in he range of approximately 200 to 800 watts per cubic centimeter, the discharge space of said envelope having an effective length several times its diameter and providing a cross section substantially filled by a wall stabilized discharge.

3. An intermediate pressure gas are lamp comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of electrodes sealed into opposite ends, an inert gas at a cold filing pressure of approximately 200 to 1250 millimeters sealed within said envelope and developing an operating pressure in the range of 1 to 10 atmospheres at volume loadings in the range of approximately 200 to 800 watts per cubic centimeter, the discharge space of said envelope having an eifective length several times its diameter and providing a cross section substantially filled by the discharge therethrough, the walls of said envelope about said discharge space operating at temperatures in the range of 1000 to 1600 C. and serving to stabilize said discharge throughout said range of volume loadings.

4. An intermediate pressure gas arc lamp for con tinuous operation comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of electrodes sealed into opposite ends, a filling of xenon within. said envelope developing an operating pressure in the range of l to 10 atmospheres at volume loadings in the range of approximately 200 to 800 watts per cubic centimeter and a light-generating efficiency of at least 10 lumens per watt, the discharge space of said envelope having an effective length several times its diameter and achieving wall stabilization of the discharge therethrough.

5. An intermediate pressure gas are lamp for continuous operation comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of electrodes sealed into opposite ends, xenon at a cold filling pressure of approximately 200 to 1250 millimeters sealed within said envelope and developing an operating pressure in the range of l to 10 atmospheres at volume loadings in the range of approximately 200 to 800 watts per cubic centimeter and a light-generating efliciency of at least 10 lumens per watt, the discharge space of said envelope having an effective length several times its diameter and providing a cross section substantially filled by a wall stabilized discharge.

6. An intermediate pressure gas are lamp comprising a tubular elongated envelope of sintered transparent high density polycrystalline alumina having a pair of metal end caps hermetically sealed into opposite ends, a pair of activated electrodes within said envelope fastened to said end caps, Xenon at a cold filling pressure of approximately to stabilize said discharge throughout said range of volume 200 to 1250 millimeters sealed within said envelope and loadings. developing an operating pressure in the range of l to 10 atmospheres at volume loadings in the range of approxi- Re erenees Cited in the file of this patent mately 200 to 800 watts per cubic centimeter, the discharge 5 UNITED STATES PATENTS space of said envelope having an effective length several times its diameter and providing a cross section substan 2,055,926 Krefit' 6, 1936 tially filled by the discharge therethrough, the Walls of 1152394 Hanlem P 1939 said envelope about said discharge space operating at 1 3 Germeshausell y 24, 1956 temperatures in the range of 1000 to 1600" C. and serving 10 ,9 ,110 Schmidt Feb. 7, 1961 

1. AN INTERMEDIATE PRESSURE GAS ARC LAMP COMPRISING A TUBULAR ELONGATED ENVELOPE OF SINTERED TRANSPARENT HIGH DENSITY POLYCRYSTALLINE ALUMINA HAVING A PAIR OF ELECTRODES SEALED INTO OPPOSITE ENDS, AN INERT GAS WITHIN SAID ENVELOPE DEVELOPING AN OPERATING PRESSURE IN THE RANGE OF 1 TO 10 ATMOSPHERES AT VOLUME LOADINGS IN THE RANGE OF APPROXIMATELY 200 TO 800 WATTS PER CUBIC CENTIMETER, THE DISCHARGE SPACE OF SAID ENVELOPE HAVING AN EFFECTIVE LENGTH SEVERAL TIMES ITS DIAMETER AND PROVIDING WALL STABILIZATION. 